IMO, it's really doubtful to be the linkage, but, in the interest of being thorough;
How does reverse work?
The reason I ask is because, to get reverse, the trans uses the L/R band, and the 3rd gear clutch; which is also called the Hi-drum, or in Moparese, the front clutch. So then, if reverse works, then we know that the Hi-drum is ok. and if that's true, ok, I concede that you could be having a linkage problem.
Then again, are you sure that it's hi that you are missing?
Tell me again what happens in "D", from a stop.. OR
If you have a tachometer, what was your rpm in "2" on the hiway? and did it take off as normal in "1"?
What I'm trying to figure out here is if the KD mechanism fell off the carb, and the trans is just upshifting really fast. or if the governor is working.